Traditional telephony was based on circuit-switched (CS) networks, which still remain the predominant means for carrying voice communication. Landline circuit-switched connections typically give rise to echo due to the imperfect balance of hybrid circuits at the junction of the 4-wire network and the 2-wire local loop. Echo is recognized as a severe voice quality impairment, and echo cancellers are typically deployed within circuit-switched networks to resolve this problem.
With the emergence and the accelerated adoption of Internet Protocol (IP) networks, it is increasingly common for voice communication paths to span a mix of IP and circuit-switched networks. Such a voice path, when terminating on a landline endpoint, will suffer degradation due to echo, requiring echo cancellation. The conventional (and typically the best) location for the echo canceller is in the circuit-switched network, near the source of the echo. However, under certain circumstances it may not be feasible to rely on deployment of echo cancellers in the circuit-switched network. For example, the network operator in charge of the IP network (or of a wireless network) may not have control over the type (and effectiveness) of echo cancellers deployed in the CS network, making it desirable or necessary to treat the echo by deploying an echo canceller in the IP network.
IP networks are subject to impairments such as packet loss and delay variation (jitter). To an echo canceller deployed in the IP network, the portion of the network between the echo canceller and the echo source is a part of the echo-path. Packet loss, jitter, as well as their remedies, i.e., packet-loss concealment (PLC) and jitter buffer delay variation (JBDV), cause nonlinear behavior of the echo-path that are extremely harmful to the operation of conventional echo cancellers designed to adapt to echo-paths with linear impulse responses.
Furthermore, PLC and JBDV can occur both in the send direction (IP-to-CS path) and the receive direction (CS-to-IP path). When PLC and/or JBDV occur in the send direction, the node including the echo canceller will not have explicit knowledge of the packet loss or JBDV experienced in that path.